If you cannot get the cable elsewhere.... What about trying cable cinch to small jack and adapter from small jack to big one. It might work, or not as cinch to jack is just ordinary wire, not well suited for SPDIF signal transfer.

To my understanding that won’t work. SPDIF is a digital signal connection but the Multiclock needs an analog signal. SPDIF coaxial uses the same connectors as analog chinch but what it transmits is completely different.
(EDIT: I didn’t see Jeff's post before)

Thanks for the help, I get an adapter to convert the digital spdif out to the analog in of the multiclock. Thanks waedi for the link! If only there was a smaller one, that doesn’t take extra power… I suppose these converter’s can’t be passive, right?

You could consider other solutions to gain more analog outputs:
- buy a high quality 2-channel converter (like RME ADI-2 series unit), feed it from SPDIF and use it as main out for the monitor speakers instead of two direct analog outputs on the Fireface UC.
- buy an ADAT converter and get eight more outputs over the optical port to feed your effect units.

Sure, there is a D/A converter inside.
If it were an RME device, you could find converter latency in the manual.
But tbh. … what latencies are you worried about?
In the worst case at single speed, maybe 1ms, if the converters are more modern, much lesser.
But that you have to ask the vendor of this box.
I doubt that they documented it, but please look at the vendors' documentation.